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Abstract:

Example systems, apparatus, circuits, and so on described herein concern
parallel transmission in MRI with on-coil current-mode (CMCD) amplifiers.
One example apparatus includes switched voltage-mode class D (VMCD)
pre-amplifiers. Another example apparatus includes amplitude modulation
of the output of the CMCD amplifiers using feedback control based, at
least in part, on a comparison of an envelope of transmit coil current to
an envelope of an input RF pulse.

Claims:

1. An apparatus, comprising: an out-of-phase signal generator to generate
first and second out-of-phase RF signals; and first and second switched
voltage-mode class-D (VMCD) amplifiers electrically coupled to the
out-of-phase signal generator, the first VMCD amplifier to amplify the
first out-of-phase RF signal and the second VMCD amplifier to amplify the
second out-of-phase RF signal, the first and second VMCD amplifiers
connected to one another in a push-pull configuration; where the first
VMCD amplifier produces an amplified first out-of-phase signal configured
to drive a first current-mode class-D (CMCD) amplifier field effect
transistor (FET), and where the second VMCD amplifier produces an
amplified second out-of-phase signal configured to drive a second CMCD
amplifier FET, the first and second CMCD amplifier FETs being connected
by a coil including an LC (inductance-capacitance) leg to define an
on-coil CMCD amplifier that controls production of an output analog radio
frequency (RF) signal by a transmit coil associated with parallel
magnetic resonance imaging (MRI) transmission.

2. The apparatus of claim 1, where the first and second VMCD amplifiers
comprise first and second FETs in a common-source configuration with
gates connected to the output of the out-of-phase signal generator and
drains connected to respective first and second VMCD amplifier voltages.

3. The apparatus of claim 1, where the out-of-phase signal generator
comprises: a comparator to amplify a digitally encoded RF pulse and
output two out-of-phase digital signals; a demodulator to demodulate the
two out-of-phase signals; and at least one differential amplifier to
amplify the demodulated out-of-phase signals; and where the amplified
demodulated out-of-phase signals correspond to the first and second
out-of-phase signals input to the first and second VMCD amplifiers.

4. The apparatus of claim 3, comprising: a feedback controller to receive
a signal indicative of an envelope of the digitally encoded RF pulse; and
an envelope detector to provide a signal indicative of an envelope of a
transmit coil current to the feedback controller; where the feedback
controller controls the on-coil CMCD amplifier to produce the RF signal
based, at least in part, on a comparison between the envelope of the
digitally encoded RF pulse and the envelope of the transmit coil current.

5. The apparatus of claim 4, where the envelope detector comprises a
current sensor coupled to the transmit coil.

6. The apparatus of claim 4, where the feedback controller modulates an
amplitude of the output analog RF signal by regulating a CMCD voltage
provided by the CMCD amplifier according to a pulse-width-modulation
(PWM) signal produced based, at least in part, on the comparison between
the envelope of the digitally encoded RF pulse and the envelope of the
transmit coil.

7. An MRI apparatus, comprising: an out-of-phase signal generator to
generate first and second out-of-phase RF signals; and first and second
VMCD amplifiers electrically coupled to the out-of-phase signal
generator, the first VMCD amplifier to amplify the first out-of-phase RF
signal and the second VMCD amplifier to amplify the second out-of-phase
RF signal, the first and second VMCD amplifiers connected to one another
in a push-pull configuration; and where the first and second VMCD
amplifiers produce amplified first and second out-of-phase signals
configure to drive an L-C switched-mode transmit coil that outputs an
analog RF signal at a desired frequency.

8. The apparatus of claim 7, where the out-of-phase signal generator
comprises: a comparator to amplify a digitally encoded RF pulse and
output two out-of-phase digital signals; a demodulator to demodulate the
two out-of-phase signals; and at least one differential amplifier to
amplify the demodulated out-of-phase signals; where the amplified
demodulated out-of-phase signals correspond to the first and second
out-of-phase signals input to the first and second VMCD amplifiers.

9. The apparatus of claim 8, comprising: a feedback controller to receive
a signal indicative of an envelope of the digitally encoded RF pulse; and
an envelope detector to provide a signal indicative of an envelope of a
transmit coil current to the feedback controller; where the feedback
controller controls the on-coil CMCD amplifier to produce the RF signal
based, at least in part, on a comparison between the envelope of the
digitally encoded RF pulse and the envelope of the transmit coil current.

10. A method, comprising: generating first and second out-of-phase
signals; amplifying the first and second out-of-phase signals with
respective first and second VMCD amplifiers; and providing the amplified
out-of-phase signals to an on-coil CMCD amplifier that drives an L-C leg
to excite an MRI transmit coil to transmit an RF signal.

11. The method of claim 10, where the out-of-phase signal is generated
by: amplifying a digitally encoded RF pulse and outputting two
out-of-phase digital signals; demodulating the two out-of-phase signals;
amplifying the demodulated out-of-phase signals; and inputting the
amplified demodulated out-of-phase signals to the VMCD amplifiers as the
first and second out-of-phase signals.

12. The method of claim 11, comprising: receiving a signal indicative of
an envelope of the digitally encoded RF pulse; determining an envelope of
a transmit coil current to the feedback controller; and controlling the
on-coil CMCD amplifier to produce the RF signal based, at least in part,
on a comparison between the envelope of the digitally encoded RF pulse
and the envelope of the transmit coil current.

13. An apparatus, comprising: an out-of-phase signal generator to
generate first and second out-of-phase RF signals based on an input RF
pulse having a desired frequency, where the first out-of-phase signal is
configured to drive a first CMCD amplifier FET, and where the second
out-of-phase signal is configured to drive a second CMCD amplifier FET,
the first and second CMCD amplifier FETs being connected by a coil
including an LC leg to define an on-coil CMCD amplifier associated with a
parallel MRI transmit coil that outputs an analog RF signal at the
desired frequency; a feedback controller to receive a signal indicative
of an envelope of the input RF pulse; and an envelope detector to provide
a signal indicative of an envelope of a transmit coil current to the
feedback controller; where the feedback controller controls the on-coil
CMCD amplifier to produce the RF signal based, at least in part, on a
comparison between the envelope of the digitally encoded RF pulse and the
envelope of the transmit coil current.

14. The apparatus of claim 13 where the out-of-phase signal generator
comprises: a comparator to amplify a digitally encoded RF pulse and
output two out-of-phase digital signals; a demodulator to demodulate the
two out-of-phase signals; at least one differential amplifier to amplify
the demodulated out-of-phase signals; and first and second switched VMCD
amplifiers to further amplify the demodulated out-of-phase signals output
by the differential amplifier; and where outputs of the VMCD amplifiers
correspond to the first and second out-of-phase signals input to the
first and second CMCD amplifiers.

15. The apparatus of claim 14 where the envelope detector comprises a
wire loop electrically coupled to the transmit coil.

16. The apparatus of claim 14 where the feedback controller modulates an
amplitude of the output analog RF signal by regulating a CMCD voltage
provided by the CMCD amplifier according to a pulse-width-modulation
(PWM) signal produced based, at least in part, on the comparison between
the envelope of the digitally encoded RF pulse and the envelope of the
transmit coil.

17. An MRI apparatus, comprising: an out-of-phase signal generator to
generate first and second out-of-phase RF signals based on an input RF
pulse having a desired frequency, where the first and second out-of-phase
signals are configured to drive an L-C switched-mode transmit coil that
outputs an analog RF signal at a desired frequency; a feedback controller
to receive a signal indicative of an envelope of the input RF pulse; and
an envelope detector to provide a signal indicative of an envelope of a
transmit coil current to the feedback controller; where the feedback
controller controls the on-coil CMCD amplifier to produce the RF signal
based, at least in part, on a comparison between the envelope of the
input RF pulse and the envelope of the transmit coil current.

18. The apparatus of claim 17 where the input RF pulse is digitally
encoded and the out-of-phase signal generator comprises: a comparator to
amplify the digitally encoded RF pulse and output two out-of-phase
digital signals; a demodulator to demodulate the two out-of-phase
signals; at least one differential amplifier to amplify the demodulated
out-of-phase signals; and first and second switched VMCD amplifiers to
further amplify the demodulated out-of-phase signals output by the
differential amplifier; and where outputs of the VMCD amplifiers
correspond to the first and second out-of-phase signals input to the
first and second CMCD amplifiers.

19. The apparatus of claim 17 where the envelope detector comprises a
current sensor coupled to the transmit coil.

20. The apparatus of claim 17 where the feedback controller modulates an
amplitude of the output analog RF signal by regulating a CMCD voltage
provided by the CMCD amplifier according to a pulse-width-modulation
(PWM) signal produced based, at least in part, on the comparison between
the envelope of the input RF pulse and the envelope of the transmit coil.

21. A method, comprising: generating first and second out-of-phase
signals based, at least in part, on a received RF pulse; receiving a
signal indicative of an envelope of the input RF pulse; determining an
envelope of an MRI transmit coil current; and controlling an on-coil CMCD
amplifier to produce an analog RF signal based, at least in part, on a
comparison between the envelope of the digitally encoded RF pulse and the
envelope of the transmit coil current.

22. The method of claim 21 where the out-of-phase signal is generated by:
amplifying a digitally encoded RF pulse and outputting two out-of-phase
digital signals; demodulating the two out-of-phase signals; amplifying
the demodulated out-of-phase signals; and inputting the amplified
demodulated out-of-phase signals to the switched mode amplifiers as the
first and second out-of-phase.

23. The method of claim 21 comprising detecting the envelope of the
transmit coil with a wire loop coupled to the transmit coil.

24. The method of claim 21 comprising modulating an amplitude of the
output analog RF signal by regulating a CMCD provided by the CMCD
amplifier according to a pulse-width-modulation (PWM) signal produced
based, at least in part, on the comparison between the envelope of the
digitally encoded RF pulse and the envelope of the transmit coil.

Description:

COPYRIGHT NOTICE

[0001] A portion of the disclosure of this patent document contains
material subject to copyright protection. The copyright owner has no
objection to the facsimile reproduction of the patent document or the
patent disclosure as it appears in the Patent and Trademark Office patent
file or records, but otherwise reserves all copyright rights whatsoever.

BACKGROUND

[0002] Magnetic resonance imaging (MRI) involves the transmission of radio
frequency (RF) energy. RF energy may be transmitted by a coil. Resulting
magnetic resonance (MR) signals may also be received by a coil. In early
MRI, RF energy may have been transmitted from a single coil and resulting
MR signals received by a single coil. Later, multiple receivers may have
been used in parallel acquisition techniques. Using multiple receivers
facilitates speeding up signal reception, which in turn may reduce scan
time. Similarly, multiple transmitters may be used in parallel
transmission techniques. Using multiple transmitters may facilitate
speeding up a transmission process, which in turn may facilitate
volumetric excitation, selective isolation, and other very high speed
features. However, conventional parallel transmission techniques have
encountered issues with scaling, fidelity, and synchronization.

[0003] Conventional systems may have attempted to parallelize their
existing RF transmitters. Thus, conventional systems may have relied on
multiple, individually powered, single channel, analog-in-analog-out RF
transmitters for parallel transmission. These systems tended not to scale
well due to cabling duplication, power transmitter duplication, control
duplication, and other issues. Even when a small number (e.g., 4) of
transmitters were employed, these systems may not have produced desired
fidelity. For example, conventional systems may have had complicated
power distribution management and may have been difficult to synchronize.
Additionally, conventional systems typically had poor isolation between
coils, resulting in degraded performance. Furthermore, these systems may
have required each element in an array to be tuned and matched, which is
a very time-consuming procedure.

[0004] Conventional systems may also have been limited by their use of
relatively low power (e.g., <50 W), low efficiency class A or class AB
amplifiers. While some systems may have included on-coil series and/or
shunt-fed class-D amplifiers, even these conventional systems have
suffered from several limitations including inadequate detuning and low
efficiency. Proposed systems that indicate that they may achieve higher
efficiency still appear to lack adequate control mechanisms. Due, at
least in part, to these limitations, conventional systems may not have
produced desired levels of amplitude and/or phase control and thus may
have had less than desirable fidelity.

[0005] U.S. Pat. No. 7,671,595 ("the '595 patent) to Griswold et al. which
issued on Mar. 2, 2010, and is entitled "On-coil Switched Mode Amplifier
for Parallel Transmission in MRI" describes an on-coil current-mode
class-D (CMCD) amplifier that may be used to produce MRI transmission
coil excitations at desired RF frequencies. The on-coil CMCD amplifier is
capable of performing within or proximate to the bore of the MRI magnet
or within less than one wavelength of the amplifier from the transmit
coil. Providing an on-coil amplifier allows digital control signals to be
sent to the coil assembly, improving synchronization between the
transmission coils while reducing interference, cross talk, physical
space requirements associated with cables, and heating normally
associated with parallel transmission MRI systems. The on-coil CMCD
amplifier described in the '595 patent is driven by signals produced by
one or more linear pre-amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate various example systems, methods,
and other embodiments of various aspects of the invention. It will be
appreciated that the illustrated element boundaries (e.g., boxes, groups
of boxes, or other shapes) in the figures represent one example of the
boundaries. One of ordinary skill in the art will appreciate that in some
embodiments one element may be designed as multiple elements, multiple
elements may be designed as one element, an element shown as an internal
component of another element may be implemented as an external component
and vice versa, and so on. Furthermore, elements may not be drawn to
scale.

[0017] Prior Art FIG. 1 illustrates a conventional parallel transmission
MRI system that used multiple independent external transmitters (e.g.,
120, 122 . . . 128), multiple transmit coils (e.g., 110, 112 . . . 118)
and multiple receive coils (e.g., 170, 172 . . . 178). Once again the
system would also have included other components (e.g., main field magnet
150, gradient coil 160, and so on). This conventional system could
perform both parallel acquisition and parallel transmission. However,
this system may have suffered from the limitations described above
including, for example, scaling, synchronization, interference between
coils, and so on. The system presented each transmit coil with an analog
signal and each transmit coil produced an analog signal. Each transmit
coil was powered by a separate power transmitter and was connected to its
power transmitter by a separate coaxial cable. The power transmitters may
have been controlled by a computer 130.

[0018] The system illustrated in FIG. 1 did not scale well due to power
transmitter proliferation, tuning and matching requirements, cable
proliferation and coupling, and power consumption increases. The sheer
volume of the multiple power transmitters and multiple cables made
physical design difficult. The power required to drive all the
transmitters and the resulting heat produced by all these transmitters
created additional design issues. Furthermore, cable paths and coil
design may have produced cross talk issues, interference issues, and so
on. Synchronization of the transmit coils was difficult, if possible, to
achieve and often involved cable length and connection engineering
issues.

[0019]FIG. 2 illustrates an example system 200 that uses multiple
independent transmit coils (e.g., 210, 212 . . . 218) and multiple
receive coils (e.g., 270, 272 . . . 278). The transmit coils illustrated
in FIG. 2 (and FIG. 3) are described in more detail in the '595 patent.
The transmit coils have on-coil switched mode amplifiers that facilitate
improved parallel transmission in MRI. The transmit coils may be powered
by digital controllers (e.g., power transmitters 220) that are controlled
by a computer 230. The transmit coils may receive a digital signal and
produce an analog signal having improved characteristics. The system also
includes other standard MRI apparatus elements (e.g., main field magnet
250, gradient coils 260, and so on).

[0020] FIG. 3 illustrates an example CMCD amplifier topology 300 similar
to the CMCD amplifier described in the '595 patent. An MRI transmit coil
configured with this topology may be referred to as an L-C-switched-mode
coil. In the illustration, the coil is represented by the series LC leg
310. The L refers to inductance in the coil 310 and the C refers to
capacitance in the coil 310. The two chokes RFC (e.g., 320, 322) act as
current sources. The drain-source capacitances CdS (e.g., 330, 332)
are in series with the coil 310. Alternative shunting of an applied DC
voltage to ground as an FET is driven to saturation produces excitation
at desired RF frequencies. Alternative shunting of an applied DC voltage
to ground as an FET is driven to saturation produces excitation at
desired RF frequencies. The signal that drives the FETs to saturation is
provided by linear circuit 380 that includes an RF transmission unit. The
coil 310 terminals are attached between the drains of two FETs (Q1 340,
Q2 342) and tuned so that the circuit is series resonant when one of the
FETs is switched on.

[0021] FIG. 4 illustrates an example CMCD topology 400 that includes an
out-of-phase signal generator 410 and a switched voltage-mode class-D
(VMCD) pre-amplification stage 420 that pre-amplifies signals provided to
an on-coil CMCD amplifier 430. It can be seen that the on-coil CMCD
amplifier 430 is similar to the CMCD amplifier shown in FIG. 3 and
includes two CMCD FETs Q5, Q6 that are driven by the output of the
pre-amplification stage 420.

[0022] The out-of-phase signal generator 410 generates two out-of-phase RF
signals and can be implemented in many ways one of which will be
described below with reference to FIGS. 5 and 7. The pre-amplification
stage 420 includes first and second VMCD amplifiers 424, 426 that are
configured to amplify one of the out-of-phase RF signals. In the
described embodiment, the first and second VMCD amplifiers include FETs
Q3, Q4 which can be MOSFETs or any other suitable switch device. The
first and second VMCD amplifiers 424 and 426 drive one of the CMCD FETs
by selectively providing a pre-amplifier voltage (Vamp) to a gate of
the driven CMCD FET. The switched mode pre-amplification stage 420 is
configured to boost the out-of-phase RF signals from the out-of-phase
signal generator 410 to a voltage level that will efficiently switch the
CMCD FETs.

[0023] FIG. 5 illustrates a CMCD topology 500 that includes a
pre-amplification stage 520 similar to the pre-amplification stage 420 of
FIG. 4. The pre-amplification stage includes first and second VMCD
amplifiers 524, 526 that drive an on-coil CMCD amplifier 530 similar to
on-coil amplifier 430 and shown schematically in block form for
simplicity. The topology 500 is configured to be driven by a digital
encoded RF pulse. The encoded RF pulse is amplified and split into two
out-of-phase signals (Q and -Q) through a high speed Emitter Coupled
Logic (ECL) comparator 512. The out-of-phase signals are demodulated
through a band pass filter 516 and further amplified through a
differential amplifier 518. The differential amplifier 518 may include a
cascade of high-speed differential amplifiers. The demodulated and
amplified out-of-phase signals are further amplified by the
pre-amplification stage 520 and after pre-amplification have sufficient
strength to drive the on-coil CMCD amplifier 530.

[0024] In one embodiment, a 0.8V peak to peak digital encoded RF pulse is
transformed through the pre-amplification stage (with a Vamp of 28V)
to a 55 V peak to peak signal, which, in many instances, will be
sufficient to efficiently switch the on-coil CMCD amplifier. Due to the
switched mode operation of the pre-amplification stage 620, the CMCD
amplifier may need to include additional components to provide amplitude
modulation for its output RF signal, as will be described in more detail
below.

[0025] FIG. 6 illustrates a CMCD topology 600 that includes an on-coil
CMCD amplifier 630 similar to CMCD amplifiers 430 and 530 (FIGS. 4 and
5). The CMCD amplifier 630 includes an amplitude modulation system that
uses feedback to modulate the amplitude of the RF signal output by the
CMCD amplifier 630. The CMCD amplifier 630 is driven by an out-of-phase
signal generator 610 that provides switching voltages of sufficient
strength to efficiently switch CMCD FETs Q9, Q10. The signals from the
out-of-phase signal generator 610 are generated based on an input RF
pulse having a desired frequency. To achieve sufficient switching
voltages, the out-of-phase signal generator 610 may include a
pre-amplification stage similar to the pre-amplification stages 420, 520
(FIGS. 4 and 5) as well as a signal generator similar to the signal
generator 510 (FIG. 5).

[0026] The topology 600 includes a CMCD amplifier feedback controller 650
to modulate the amplitude of the output signal of the on-coil CMCD
amplifier 630. The feedback controller 650 receives signals indicative of
a transmit coil current from a transmit coil current sensing unit 640.
The feedback controller 650 also receives signals indicative of the input
RF pulse from the out-of-phase signal generator 610. The feedback
controller 650 compares the signals indicative of the transmit coil
current to the signals indicative of the input RF pulse and modulates an
amplitude of the output of the CMCD amplifier 630 based, at least in
part, on this comparison.

[0027] FIG. 7 illustrates a CMCD amplifier topology 700 that includes an
on-coil CMCD amplifier 730 similar to CMCD amplifier 630 (FIG. 6) and an
out-of-phase signal generator 710 similar to the out-of-phase signal
generator 510 of FIG. 5 and a VMCD pre-amplification stage 720 similar to
the VMCD pre-amplification stages 420, 520 of FIGS. 4 and 5,
respectively. The pre-amplification stage 720 includes first and second
VMCD amplifiers 724, 726 that drive an on-coil CMCD amplifier 730 similar
to on-coil amplifiers 430 and 630 (FIGS. 4 and 6).

[0028] The topology 700 is configured to be driven by a digital encoded RF
pulse. The encoded RF pulse is amplified and split into two out-of-phase
signals (Q and -Q) through a high speed Emitter Coupled Logic (ECL)
comparator 712. The out-of-phase signals are demodulated through a band
pass filter 716 and further amplified through a differential amplifier
718. The differential amplifier 718 may include a cascade of high-speed
differential amplifiers. The demodulated and amplified out-of-phase
signals are further amplified by the pre-amplification stage 720 and
after pre-amplification have sufficient strength to drive the on-coil
CMCD amplifier 730.

[0029] The topology 700 includes a feedback controller 750 similar to the
feedback controller 650 of FIG. 6. The feedback controller 750 modulates
the amplitude of the output signal from the on-coil CMCD amplifier 730
based on the envelope of the input digital encoded pulse as compared to
the envelope of the current flowing in the transmit coil. The feedback
controller 750 includes a demodulator that demodulates a signal
indicative of an envelope of the digital encoded RF pulse and inputs the
envelope to an error amplifier 765. A current envelope sensor 740 is
coupled to the transmit coil and provides an envelope of the transmit
coil current to an error amplifier 765. The current envelope sensor 740
may be implemented by coupling a wire loop to the transmit coil and
passing the demodulated signal through a low-pass filter with a cutoff
frequency below the coil resonant frequency (e.g., 63.6 MHz at 1.5 T
field strength).

[0030] An output of the error amplifier 765 is provided to a comparator
770. The comparator 770 combines the output of the error amplifier with
an output of a saw oscillator 790 and thus acts as an oscillator to
generate a pulse width modulated (PWM) signal based on the comparison of
the input RF signal envelope and the envelope of the transmit coil
current. This PWM signal is used to control a modified buck converter 780
connected to the power stage for the CMCD amplifier. The buck converter
780 modulates the amplitude of the RF signal output by the CMCD amplifier
730. A trigger signal is sent to the saw oscillator 790 to avoid any
false switching when no RF pulse is present. In this manner, the feedback
controller 750 modulates an amplitude of the output of the CMCD amplifier
based, at least in part, on a comparison between envelopes of the input
RF pulse and the transmit coil current. Sensing and comparing signal
envelopes rather than sensing and comparing the signals themselves is
less complex, which facilitates topology 700 providing improved
performance over other systems.

[0031] FIG. 8 illustrates a method 800 associated with parallel
transmission MRI. The method includes, at 810, generating first and
second out-of-phase signals. At 820 the first and second out-of-phase
signals are amplified with respective first and second VMCD amplifiers.
At 830 the amplified out-of-phase signals are provided to an on-coil CMCD
amplifier that drives an L-C leg to excite an MRI transmit coil to
transmit an RF signal.

[0032] To generate the out-of-phase signals, the method may include
amplifying a digitally encoded RF pulse and outputting two out-of-phase
digital signals, demodulating the two out-of-phase signals, amplifying
the demodulated out-of-phase signals, and inputting the amplified
demodulated out-of-phase signals to the VMCD amplifiers as the first and
second out-of-phase signals. The method may also include receiving a
signal indicative of an envelope of the digitally encoded RF pulse,
providing a signal indicative of an envelope of a transmit coil current
to the feedback controller, and controlling the on-coil CMCD amplifier to
produce the RF signal based, at least in part, on a comparison between
the envelope of the digitally encoded RF pulse and the envelope of the
transmit coil current.

[0033] FIG. 9 illustrates a method 900 associated with parallel
transmission MRI. The method includes, at 910, generating first and
second out-of-phase signals based, at least in part, on a received RE
pulse. The method also includes, at 920, receiving a signal indicative of
an envelope of the input RF pulse. At 930 an envelope of an MRI transmit
coil current is determined. At 940 the on-coil CMCD amplifier is
controlled to produce an analog RF signal based, at least in part, on a
comparison between the envelope of the input RF pulse and the envelope of
the transmit coil current modulating an amplitude of the output analog RF
signal. The controlling of the CMCD amplifier may be performed by
regulating a CMCD voltage provided by the CMCD amplifier according to a
pulse-width-modulation (PWM) signal produced based, at least in part, on
the comparison between the envelope of the input RF pulse and the
envelope of the transmit coil.

[0035] The apparatus 1000 includes a basic field magnet(s) 1010 and a
basic field magnet supply 1020. Ideally, the basic field magnets 1010
would produce a uniform B0 field. However, in practice, the B0
field may not be uniform, and may vary over an object being imaged by the
MRI apparatus 1000. MRI apparatus 1000 may include gradient coils 1030
configured to emit gradient magnetic fields like GS, GP and
GR. The gradient coils 1030 may be controlled, at least in part, by
a gradient coils supply 1040. In some examples, the timing, strength, and
orientation of the gradient magnetic fields may be controlled, and thus
selectively adapted during an MRI procedure.

[0036] MRI apparatus 1000 may include a set of RF antennas 1050 that are
configured to generate RF pulses and to receive resulting magnetic
resonance signals from an object to which the RF pulses are directed. In
one example, the RF antennas 1050 may be considered to correspond, at
least in part, to element L-C leg 310 (FIG. 3). In some examples, how the
pulses are generated and how the resulting MR signals are received may be
controlled and thus may be selectively adapted during an MRI procedure.
Separate RF transmission and reception coils can be employed. The RF
antennas 1050 may be controlled, at least in part, by a set of RF
transmission units 1060. An RF transmission unit 1060 may provide a
signal to a CMCD amplifier 1062, which may manipulate the signal and
provide a different signal to the RF antenna 1050. The signal may be
manipulated (e.g., amplified) according to approaches described above in
connection with FIGS. 3-7.

[0037] The gradient coils supply 1040 and the RF transmission units 1060
may be controlled, at least in part, by a control computer 1070. In one
example, the control computer 1070 may be programmed to perform methods
like those described herein. The magnetic resonance signals received from
the RF antennas 1050 can be employed to generate an image, and thus may
be subject to a transformation process like a two dimensional FFT that
generates pixilated image data. The transformation can be performed by an
image computer 1080 or other similar processing device. The image data
may then be shown on a display 1099. While FIG. 10 illustrates an example
MRI apparatus 1000 that includes various components connected in various
ways, it is to be appreciated that other MRI apparatus may include other
components connected in other ways.

[0038] In one example, MRI apparatus 1000 may include control computer
1070 and a digital controller operably connected to the CMCD amplifiers
1062. The CMCD amplifiers 1062 may include a set of L-C-switched-mode
coils operably connected to the digital controller. In one example, a
member of the set of L-C-switched-mode coils may be individually
controllable by the control computer 1070. Additionally, the control
computer 1070 may provide an L-C-switched-mode coil with a digital
control signal and the L-C-switched-mode coil may output an analog RF
signal based, at least in part, on the digital control signal.

[0039] In one example, the set of L-C-switched mode coils may be operably
connected to the control computer 1070 by dedicated connections. The
dedicated connections may include be a copper cable, a fiber optic cable,
a wireless connection, and so on. In one example, the L-C-switched-mode
coil may be operably connected to a local memory that stores bit patterns
that control production of the analog RF signal. Thus, the digital
control signal may identify a stored bit pattern.

[0040] To the extent that the term "or" is employed in the detailed
description or claims (e.g., A or B) it is intended to mean "A or B or
both". The term "and/or" is used in the same manner, meaning "A or B or
both". When the applicants intend to indicate "only A or B but not both"
then the term "only A or B but not both" will be employed. Thus, use of
the term "or" herein is the inclusive, and not the exclusive use. See,
Bryan A. Garner, A Dictionary of Modern Legal Usage 624 (2d. Ed. 1995).

[0041] To the extent that the phrase "one or more of, A, B, and C" is
employed herein, (e.g., a data store configured to store one or more of,
A, B, and C) it is intended to convey the set of possibilities A, B, C,
AB, AC, BC, and/or ABC (e.g., the data store may store only A, only B,
only C, A&B, A&C, B&C, and/or A&B&C). It is not intended to require one
of A, one of B, and one of C. When the applicants intend to indicate "at
least one of A, at least one of B, and at least one of C", then the
phrasing "at least one of A, at least one of B, and at least one of C"
will be employed.